Only do restricted join rules signature checks for room versions 8/9. (#10927)

Otherwise the presence of a (bogus, unused) field could cause
auth checks to fail.

changelog.d/10927.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse v1.40.0 where the signature checks for room version 8/9 could be applied to earlier room versions in some situations.

synapse/event_auth.py

@@ -113,7 +113,8 @@ def check(
                 raise AuthError(403, "Event not signed by sending server")
 
         is_invite_via_allow_rule = (
-            event.type == EventTypes.Member
+            room_version_obj.msc3083_join_rules
+            and event.type == EventTypes.Member
             and event.membership == Membership.JOIN
             and "join_authorised_via_users_server" in event.content
         )